Combustion of a propellant in a rocket motor creates a hostile environment that is characterized by extremely high temperatures, pressures, and turbulence. Combustion temperatures within the rocket motor typically exceed 5000° F. (2760° C.) and pressures frequently exceed 1500 pounds per square inch (“psi”) (10.3 MPascal). In addition, velocities of gases produced during the combustion reach or exceed Mach 0.2 near the throat of the rocket motor. To protect the rocket motor as the propellant is combusted, heat-sensitive components of the rocket motor are insulated. Without insulation, the combustion gases would erode and burn through the heat sensitive components, causing the rocket motor to fail.
The rocket motor is typically insulated with a material that provides resistance to high temperatures, pressures, and erosive flow conditions. Various insulative materials have been used, such as phenolic resins, epoxy resins, high temperature melamine-formaldehyde coatings, ceramics, or polyester resins. However, these materials, when cured, become rigid and are essentially unworkable, which makes construction of the rocket motor and processing of these materials difficult. In addition, structures formed from these materials crack or blister when exposed to the rapid temperature and pressure changes that occur as the propellant is burned.
Other insulative materials include an elastomeric polymer that is reinforced with asbestos, polybenzimidazole fibers, or polyaramid fibers. These insulative materials are “ablative” in that they are partially consumed during combustion of the propellant. While the insulative material gradually erodes, the insulative material remains in the rocket motor for a sufficient amount of time to protect the rocket motor while the propellant completely combusts. In other words, the insulative material erodes at a sufficiently slow rate that adequate protection is provided during the operation of the rocket motor.
One insulative material that has been widely used in rocket motors includes asbestos and a nitrile butadiene rubber (“NBR”). For instance, an asbestos-filled nitrile butadiene rubber (“ASNBR”) composition has been used in the Reusable Solid Rocket Motor (“RSRM”) space shuttle program. When used as an insulative material, the ASNBR composition provides excellent ablative characteristics and good rheological, mechanical, and electrostatic properties. However, due to environmental and health concerns with using asbestos and the closure of asbestos mines in Canada, an asbestos-free replacement for ASNBR is actively being sought. To date, the most suitable replacements for the ASNBR composition include a 7% KEVLAR®-filled ethylene propylene diene monomer (“EPDM”) material, an 11% KEVLAR®-filled EPDM material, and a carbon-filled EPDM material, all of which are produced by Alliant Techsystems Inc. (Edina, Minn.).
Additional EPDM compositions have been used to insulate rocket motors. United States Patent Application Publication 2002/0018847 to Guillot and U.S. Pat. No. 6,691,505 to Harvey et al. disclose, inter alia, a rocket motor insulation that includes carbon fibers dispersed in an EPDM matrix. The insulation also includes polybenzoxazole fibers (“PBO”), polybenzimidazole fibers, aramid fibers, iron oxide, milled glass, talc, silica, or ceramic clay.
United States Patent Application Publication 2002/0142147 to Sogabe et al. discloses a transmission belt that includes a rubber belt body and a cord, which are integrated by vulcanizing the rubber of the belt body. The belt body is formed from chloroprene rubber or a hydrogenated nitrile rubber and the cord is formed from fibers of poly-p-phenylene-benzobisoxazole. Poly-p-phenylene-benzobisoxazole is also known as poly-p-phenylene-2,6-benzobisoxazole or polybenzoxazole. Therefore, as used herein “PBO” refers to poly-p-phenylene-benzobisoxazole, poly-p-phenylene-2,6-benzobisoxazole, or polybenzoxazole. U.S. Pat. No. 5,891,561 to Kinoshita et al. also discloses a transmission belt having a fiber cord formed from aramid fibers or PBO fibers. The fiber cord is embedded in rubber, such as a hydrogenated nitrile rubber, a nitrile butadiene rubber, chlorophene rubber, chlorosulfonated polyethylene rubber, or alkylated chlorosulfonated polyethylene rubber.